Over swinging is very common among players striving for more distance. Mathematically, a longer arc should create more club head speed. Not so - unless done correctly. Over swinging will definitely throw the club head in a very poor plane swing.
How far should you take the club back? Only as far as you can turn your shoulders, with balance. If the club goes back farther than the shoulders, the left wrist will break down. This will not only open or close the clubface but will take the pulling action away from the legs, thus allowing the shoulders or hands, or both, to take over and dominate the forward swing. This type of motion puts the club in an outside-in plane (over the top).
How does a player cultivate the proper length of back swing? We are all individuals and our muscle coordination is not the same; therefore it would be foolish to try to force the club to a parallel position at the top of the back swing. Trying to take the club head to parallel will not only shorten your distance, it will wreck your accuracy as well. So returning to the earlier premise: The club should not go back any further than you can turn your shoulders.
How can golfers learn this? By feel. Keep the left wrist in a flat position in relation to the back of the left forearm and back of left hand. This will allow the wrist to hinge naturally, not break, which will allow the club to go back only as far as the shoulders turn. Practice this drill daily, and before you know it your muscles will get conditioned and trained to stretch further under control to create the arc you desire without over swinging.
Going to parallel is not the answer to solid shot making and power. Distance is the speed of the lower body pulling the speed of the motion of your left arm corresponding together to launch the ball towards the target.